1. Field of the Invention
This invention relates to an apparatus for purifying exhaust gas from a multi cylinder internal combustion engine and in particular is related to an ignition system for a multi cylinder internal combustion engine.
2. Description of the Prior Art
Recently, as is well known, air pollution caused by exhaust gas from internal combustion engines has increased resulting in serious social problems. In order to reduce the pollutants in exhaust gases, various types of apparatus for purifying exhaust gases have been proposed. Each proposed apparatus has some defect such as, inadequate purifying performance, increased size and complexity of the apparatus, and the like.
Vehicle engines are generally designed so that the air-fuel ratio in each of the cylinders is kept as uniform as possible. However, in the exhaust gas, the concentration of nitrogen oxides (referred to as NOx hereinafter) is high at the air-fuel ratio at which the fuel consumption is minimized under a partial load condition. On the other hand, the concentrations of both of carbon monoxide (referred to as CO hereinafter) and hydrocarbons (referred to as HC hereinafter) are high when the throttle is opened near its extreme position because of the low air-fuel ratio providing maximum engine output. Also, since the air-fuel mixture is incompletely burned in the cylinder while the engine runs at lower speed due to reasons such as low temperature of the inside wall of the cylinder, exhaust gases containing unburned components such as CO and HC are produced. Therefore, conventional multi cylinder internal combustion engines suffer from the defect that one or more concentrations of NOx, CO and HC in the exhaust are increased under almost any running condition of the engines.
As is well known, the concentrations of the CO and HC can be reduced by the effective combustion thereof at higher temperatures with sufficient air charges, but such conditions increase the concentration of the NOx. In order to reduce the NOx concentration, the combustion temperature, and concomittantly the engine efficiency, should be lowered. One such approach includes an exhaust gas recycling system wherein the exhaust gas is partially diverted to the intake system. However, this approach has defects. Since the combustion becomes unstable without additional fuel charges, additional fuel is added simultaneously with the recirculation of the exhaust gas by the operation of enriched air-fuel mixture apparatus. However, as it is required to control the unburned gas, including the residual gas, in the combustion chamber at about a constant ratio, the apparatus for the latter method can not be simplified in structure and is expensive.